1. Field of the Invention
The present invention relates to a magnetic shield member serving as means for preventing intrusion of an outer magnetic field and a magnetic shield structure and a magnetic memory apparatus using this magnetic shield member.
2. Description of the Related Art
In recent years, in an increasingly worsening electromagnetic environment, since magnetic failures also increases in the field of a static magnetic field, there is a demand for a high-performance, simple, and inexpensive magnetic shield method.
For the purpose of protecting electronic devices from a leak magnetic field from a superconductive applied device or preventing an electron beam from deflecting due to a magnetic field in a device using the electron beam, sufficient magnetic shield is important. In particular, in the field of magnetic recording, the magnetic shield is attached more importance. It is impossible to use a magnetic head for audio, in which a high permeability material such as Permalloy is used, without the magnetic shield. Moreover, in accordance with the spread of magnetic recording media such as magnetic recording disks like a flexible disk and a hard disk and magnetic recording cards such as a cash card and a credit card, the magnetic shield is required for the purpose of protection of information from an outer magnetic field.
A memory attracting attentions as a high-speed, large capacity (highly integrated), low power consumption, and small nonvolatile memory is a magnetic memory called Magnetic Random Access Memory (MRAM), which is described in, for example, Wang et al., IEEE Trans. Magn. 33 (1997), 4498. This memory is attracting attentions due to the improvement in characteristics of Tunnel Magnetoresistance (TRM) materials in recent years.
The MRAM is a semiconductor magnetic memory utilizing a magnetoresistance effect based on a spin dependent conduction phenomenon peculiar to a nano-magnetic substance and is a nonvolatile memory that can keep memory without the supply of electric power from the outside. Due to its simple structure, it is easy to highly integrate the MRAM. In addition, the MRAM has a large number of rewritable times because recording is performed by rotation of a magnetic moment. Further, concerning an access time, it is expected that the MRAM operates at an extremely high speed. It has already been reported in R. Scheuerlein et al., ISSCC Digest of Technical Papers, pp. 128-129, February 2000 that the MRAM is capable of operating at 100 MHz.
To explain such an MRAM more in detail, as schematically shown in FIG. 20, a TMR element 10 serving as a memory element of a memory cell of the MRAM is a memory element consisting of a structure obtained by stacking a magnetization fixed layer 26 with a direction of magnetization fixed and a storage layer 2, in which magnetization rotates relatively easily, via a tunnel barrier layer 3.
A ferromagnetic substance consisting of nickel, iron, or cobalt or an alloy of these metals is used for the storage layer 2 and the magnetization fixed layer 26. The tunnel barrier layer 3 consists of an insulator such as an oxide or a nitride of aluminum, magnesium, silicon, or the like. The tunnel barrier layer 3 plays roles for cutting magnetic coupling of the storage layer 2 and the magnetization fixed layer 26 and feeding a tunnel current.
Although not shown in the figure, the magnetization fixed layer 26 has a first magnetization fixed layer and a second magnetization fixed layer. A conductive layer of ruthenium, copper, chrome, gold, or silver, to which these magnetization fixed layers couple ferromagnetically, may be sandwiched between both the magnetization fixed layers. In addition, the second magnetization fixed layer is in contact with an antiferromagnetic substance layer of a manganese alloy of iron, nickel, platinum, iridium, or rhodium, a cobalt oxide, or a nickel oxide. Thus, the second magnetization fixed layer has a strong magnetic anisotropy in one direction due to an exchange interaction that acts between these layers.
In the memory cell, an n-type field effect transistor for readout 19 consisting of a gate insulation film 15, a gate electrode 16, a source region 17, and a drain region 18, which is formed, for example, in a p-type well region formed in a p-type silicon semiconductor substrate 13, is arranged. A word line for writing 12, a TMR element 10, and a bit line 11 are arranged above the n-type field effect transistor for readout 19. The TMR element 10 is connected to the bit line 11 via an uppermost conductive layer. The word line 12 is provided below the TMR element 10 via an insulating layer. A sense line 21 is connected to the source region 17 via the source electrode 20. The field effect transistor 19 functions as a switching element for readout. A wiring for readout 22, which is drawn out from a space between the word line 12 and the TMR element 10, is connected to the drain region 18 via a drain electrode 23. Note that the transistor 19 may be an n-type or p-type field effect transistor. Besides, various kinds of switching elements such as a diode, a bipolar transistor, and a metal semiconductor field effect transistor (MESFET) can be used as the transistor 19.
FIG. 21 shows an equivalent circuit diagram of the MRAM. The MRAM has the bit lines 11 and the word lines for writing 12 that cross each other. At crossing points of these writing lines, the storage elements 10 as well as the field effect transistors 19, which are connected to the storage elements and select an element at the time of readout, and the sense lines 21 are provided. The sense lines 21 are connected to sense amplifiers 23 and detect stored information. Note that reference numeral 24 in the figure denotes bidirectional word line current drive circuits for writing and 25 denotes bit line current drive circuits.
FIG. 22 is an asteroid curve indicating writing conditions for the MRAM. FIG. 22 shows a reversed threshold value in a storage layer magnetization direction according to a magnetic field in a direction of easy axis of magnetization HEA and a magnetic field in a direction of hard axis of magnetization HHA applied to the MRAM. When a corresponding synthetic magnetic field vector is generated outside this asteroid curve, magnetic field reversal is caused. However, a synthetic magnetic field vector inside the asteroid curve never reverses the cell from one side of a current bistable state thereof. In addition, in the cell other than the crossing points of the word lines and the bit lines to which an electric current flows, since magnetic fields, which occur only in the word lines or the bit lines, are applied. Thus, when a magnitude of the magnetic fields is equal to or larger than a one direction reversed magnetic field HK, a direction of magnetization of the cells other than the crossing points are also reversed. Therefore, an arrangement is made such that a selected cell is made selectively writable only when a synthetic magnetic field is in a gray area in the figure.
As described above, in the MRAM, in general, two writing lines of the bit line and the word line are used, whereby asteroid magnetization reversal characteristics is utilized to write information only in a designated memory cell according to reversal of a magnetic spin. Synthetic magnetization in a single storage area depends upon vector synthesis of the magnetic field in a direction of easy axis of magnetization HEA and the magnetic field in a direction of hard axis of magnetization HHA applied to the storage area. A writing current flowing through the bit line applies the magnetic field in a direction of easy axis of magnetization HEA to the cell. A current flowing through the word line applies the magnetic field in a direction of hard axis of magnetization HHA to the cell.
FIG. 23 explains a readout operation of the MRAM. As described above, in writing of information, a magnetic spin of a cell is reversed by synthetic magnetic fields in the crossing points of the bit lines 11 and the word lines 12, which are wired in a matrix shape, and directions of the magnetic spin are recorded as information of “1” and “0”. In addition, the readout is performed utilizing a TMR effect that is an application of the magnetic resistance effect. The TMR effect is a phenomenon in which a resistance value changes depending upon a direction of a magnetic spin. “1” and “0” of the information are detected according to a state of a high resistance in which the magnetic spin is anti-parallel and a state of a low resistance in which the magnetic sin is parallel. This readout is performed by feeding a readout current (tunnel current) between the work lines 12 and the bit lines 11 and reading out an output according to a level of the resistance to the sense lines 21 via the field effect transistors for readout 19 described above.
As described above, the MRAM is expected as a high speed and nonvolatile large capacity memory. However, since a magnetic substance is used for keeping a memory, there is a problem in that information is erased or rewritten due to an influence of an outer magnetic field. “0” and “1” are written by rotating a direction of a magnetic spin 180° and are read out according to a difference of resistance caused by the direction of the magnetic spin. However, since a coercive force (Hc) is, for example, about several Oe to 100 Oe, if an internal leak magnetic field due to an outer magnetic field exceeding the coercive force acts, it may be impossible to selectively perform writing in a predetermined memory cell.
Therefore, as a step for putting the MRAM to practical use, establishment of measures against an outer magnetic field, that is, a magnetic shield structure for shielding an element from external electromagnetic waves is desired.
As a shape and a structure of magnetic shield means, in general, a space to be prevented from being affected by magnetic fields are covered and surrounded by a high magnetic permeability material or a high saturation magnetization material. In addition, when a space to be prevented from being affected by magnetic fields is small, it is also possible to sandwich the space with two magnetic shield plates (a sandwich structure).
As a magnetic shield structure of the MRAM, there is a proposal for giving magnetic shield characteristics to the MRAM by using an insulative ferrite (MnZn and NiZn ferrite) layer for a passivation film of an MRAM element (see U.S. Pat. No. 5,902,690 (fifth column and FIGS. 1 and 3)). In addition, there is a proposal forgiving a magnetic shield effect to the MRAM by attaching high magnetic permeability magnetic substances such as permalloy above and below a package to prevent intrusion of magnetic fluxes into an internal element (see U.S. Pat. No. 5,939,772 (second column and FIGS. 1 and 2)). Moreover, there is a disclosure about a structure in which an element is covered with a shield lid of a magnetic material such as soft iron (JP-A-2001-250206 (right column on page 5 and FIG. 6)).
In order to prevent intrusion of external magnetic fluxes into the memory cell of the MRAM, it is most important to surround an element with a magnetic material having a high magnetic permeability to provide a magnetic path that prevents magnetic fluxes from intruding into the MRAM. For this purpose, best means is to completely cover the element with a magnetic shield layer, but it is difficult to manufacture an actual shield structure. Thus, a magnetic shield that can be manufactured easily is desired.
Thus, as Japanese Patent Application No. 2002-357806, the applicant has already proposed an MRAM that is simply provided with a magnetic shield layer and can realize a high-performance magnetic shield effect. This will be hereinafter referred to as the invention of the earlier application.
According to this invention of the earlier application, as shown in FIGS. 24A, 24B and 20, in a package in which a magnetic random access memory (MRAM) 30 consisting of a memory element formed by stacking the magnetization fixed layer 26 with a direction of magnetization fixed and the magnetic layer 2, in which a direction of magnetization can be changed, via the tunnel barrier layer 3 as described above is sealed by a sealing material 32 such as resin, magnetic shield layers (magnetic shield plates) 52A and 52B having a rectangular parallelepiped shape as a planar shape for magnetically shielding the MRAM 30 are provided in contact with one outer surface and/or the other outer surface of the sealing material 32 (or on at least one side of the sealing material 32 in the inside thereof in a non-contact state with the MRAM 30). Note that, in the figure, reference numeral 41 denotes a die pad for fixing the MRAM 30 and 31 denotes an external lead. However, for example, wire bonding for connecting the MRAM 30 and the external lead 31 is not shown in the figure.
Therefore, according to the magnetic memory device of the invention of the earlier application, taking notice of the fact that the MRAM 30 is mainly used as the package molded by the sealing material 32 such as resin, the magnetic shield layers 52A and 52B are pasted to one outer surface (e.g., an upper surface of the package on a chip surface side of the memory element) or the other surface (e.g., a lower surface of the package on a chip rear surface side of the memory element) of the molded package sealing material 32 substantially in parallel with the MRAM 30 or an outer magnetic field (a magnetic line of force) with an adhesive or the like. This makes it possible to easily attach or detach the magnetic shield layers 52A and 52B, which are processed in a shape effective for magnetic shield, or it is possible to easily embed the magnetic shield layers 52A and 52B in a predetermined position in the sealing material 32 on at least one side of the memory element 30 simply by arranging the magnetic shield layers 52A and 52B in a mold at the time of molding. Therefore, it is possible to easily realize magnetic shield, which is high-performance for the MRAM, and simplify work for mounting the magnetic shield. In addition, this package has a structure and a shape that are also suitable when the package is mounted on a circuit board.
The magnetic shield effect realized by using the magnetic material is easily understood with reference to FIGS. 25A and 25B. FIG. 25A shows a state in which a magnetic substance 51 of a ring shape in section, which has a hollow 50 in the inside, is placed in an outer magnetic field H0. The magnetic substance 51 is affected by the magnetic field and magnetized to have magnetic poles. The magnetic poles generated in the magnetic substance generate a magnetic field in a direction opposite to a direction of the outer magnetic field around the magnetic poles. When this magnetic field in the opposite direction and the magnetic field around the magnetic poles are synthesized, as shown in FIG. 25B, a space in the inside surrounded by the magnetic substance 51 changes to a very small magnetic field space having an inner magnetic field Hi. This means that the magnetic substance 51 has caused the shield effect.
However, with the structure in which mainly a space to be prevented from being affected by a magnetic field is covered and surrounded by the magnetic substance 51 consisting of a high magnetic permeability material or a high saturation magnetization material as described above, the MRAM is not preferably mounted on a device that tends to be reduced in size and weight. In addition, when a space to be prevented from being affected by a magnetic field is small, it is possible to sandwich the space with two magnetic shield plates (a sandwich structure). However, when mounting of the MRAM on a device and weight of the device are taken into account, it is indispensable to form a magnetic shield plate in a shape and a structure that realizes larger shield efficiency with a smaller volume.
In particular, magnetic saturation starts when the outer magnetic field H0 increases in size to be close to a limit of saturation magnetic flux density of the magnetic shield material. The magnetic saturation starts from a part where magnetic lines of force concentrate in the inside of a magnetic shield member and a magnetic permeability decreases. As a result, the magnetic shield effect falls. Consequently, it is also necessary to relax the magnetic saturation in the inside of the magnetic shield member.
It is generally recognized that magnetic lines of force are substantially perpendicular to a high magnetic permeability material in a boundary of the air and the high magnetic permeability material, a density of magnetic lines of force (magnetic flux density) in a magnetic substance increases, and a density of magnetic lines of force in a space surrounded by the magnetic substance decreases. Taking this recognition into account, in the case of a planar magnetic shield plate 52 of a rectangular shape (e.g., a square shape) as shown in FIG. 26, a portion (c), where magnetic lines of force concentrate, is generated in the magnetic substance due to a magnetic field intruding from a facet portion (e) ((e) in FIG. 26) perpendicular to a direction of an outer magnetic field and a facet portion (b) parallel to the magnetic shield plate 52. As a result, magnetic saturation tends to occur and the magnetic shield effect falls.